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# A race to keep pace!

Video transcript

So we've talked
about pacemaker cells and I thought it would be
kind of neat to draw out again exactly what these pacemaker
cells do and compare them to one another. So we know we have different
types of pacemaker cells. And I'm going to use our
millivolt scale just as we usually do to kind
of compare them. So the first type
is our SA node. So let me put that up here. SA node. And those cells
start out negative and then they kind of
slowly creep up positive. And try to remember
why that happens. The main reason for
that is that you have the increasing
sodium permeability. So at this point, more sodium is
kind of rushing into the cell, sodium is getting in. And if it's getting
in quickly, let's say it's like just gushing in, then
that line would be very, very steep, you know? Something like this. But if it was kind of
getting in very slowly, it would be a little
bit more shallow. So I've drawn it kind of the
way that it usually is drawn. Kind of somewhere in between. And when that sodium
permeability hits or when that cell hits
a certain threshold, let's say somewhere in there. Then it's going to fire an
action potential, right? So this is our
threshold for firing. And that just means that the
channels for calcium open up and so they open up and
make the cell go positive. And then eventually, the
potassium channels take over and it becomes negative again. And this part
right here then, we think of as the
action potential. So this is what it looks
like for the SA node. But let me now do it
again for the AV node. So if there's an AV node,
it's going to kind of get up to that threshold a
little bit more slowly, but once it gets
there, it's going to look basically the
same as the SA node. Really no different. And it's going to
come down again. So this might be the AV node. And finally, we have,
let's say, a cell that takes even longer
to get to threshold. This would be like
the bundle of His and, again, it looks
basically the same once it does get there though. So these action
potentials don't really look that different from one
another, but the amount of time it takes to get to
threshold changes because the bundle of His
cells, for example, they are going to be the least
permeable to sodium. An AV node would be
somewhere in between and the SA node are most
permeable to sodium. So that's why those
lines are slightly less steep as you go along. So this is how it looks
and the key difference here is that you're basically
extending this heartbeat out, right? This is one heartbeat and
this would be like the SA node heartbeat, but if your AV node
was controlling your heartbeat, it might take a little bit
longer, something like that. And if your bundle of His was
taking control of your heart, the heartbeat would be really
long, something like that. So it would take
longer and longer for the heartbeat
in terms of time. Depending on which part of the
electrical connection system is in charge. So let's actually think
about that a little bit more carefully. So let's say we have
our SA node and you're talking about heartbeats. Let's actually write out,
let's say, heart rate, heart rate is HR. And this is going to
be in beats per minute. And then let's say I
flipped it around and wanted to know how long one beat takes. So one beat. And that will probably
have to be done in seconds. So how would that
be for the SA node? Well, we know that the
SA node, and this is just a number out of books, you
can find them saying something like 60 to 90 beats per minute. And if we took the upper
range, let's say you took 90, and try to figure out how
long one beat would take, well, you'd say, OK. You have one minute gives you
90 beats, I'll put B for beats. And then you have one, let's say
minute, is 60 seconds, right? And so the minutes cancel. And now you're left with, the
zeros cancel, 2/3 of a second. Right? So 2/3 of a second per beat. And actually, I might even
like to I'm going to erase 2/3 and just rewrite that as 0.66. OK? It's 0.66. Something like that So that's how long it
takes for the SA node to fire off one beat. And, in fact, just to really
hammer home the point, that's this distance,
right here, right? That's 0.66 seconds. So now for the AV node, we
could do the exact same thing. We could say, well, the
AV node, we know usually is somewhere between
40 to 60 beats. And I'm going to
use that number. And this one's
really easy, right? Because if it's
60 beats a minute, that means that one
beat is 1 second. So that was actually
a really quick one. So that's 1 second. And finally for
the bundle of His, I'm going to write
that as BoH again. Bundle of His is going to
be somewhere between let's say 20 to 30 beats per minute. And if we use the
number 30, that means that you get a
beat every 2 seconds. So every 2 seconds,
this will go off. And I know that my picture now,
since you know those numbers, it's not going to
look as impressive. Because I should have drawn
the bundle of His even more stretched out than
it is, but just assume that that's two
seconds on that graph. So if that's the case,
now let's kind of jump back to how we usually
think about our heart. And the fact that you've
got four chambers, right? And the conduction
system is actually going to go through all of that. And starts here in the SA
node, goes down to the AV node, and then you've got the bundle
of His somewhere down there. And you've got
connections down there. And you might be thinking,
well, wait a second, you haven't drawn
in all of the rest of the electrical conduction
system, and that's true, but for right now, let's just
focus on these three parts. Right? So you've got AV here, and
you've got the bundle of His over here, BoH. So you've got these three parts. And they're kind of spaced out. Right? Like this is 2 centimeters
apart, let's say. I'm just kind of guesstimating. This might be even closer,
let's say one centimeter. So these are kind
of anatomically how they're laid out. In terms of how far apart
they are from each other. So the question might
come up, how exactly do you explain the
fact that it's always the SA node that fires off? Right? It's never your AV node
or your bundle of His. We always say, well, he's
in sinus rhythm, right? And what does that mean? Well, if someone says
someone is in sinus rhythm, all they're saying
is that the SA node is what's controlling
their rhythm. So sinus rhythm, you might
hear that actually a lot on TV shows, I've noticed, they like
to throw that term around. And it just means that you're in
a rhythm controlled by your SA node. So how does that work exactly? Because if it's firing every
0.66 seconds, that's fine, but how come these two other
pacemaker cells aren't ever firing? Well, it gets back to basically
trying to beat them out. So if you can get a
signal from your SA node, this is, let's say, your
SA node from that drawing above, if you can get
it to your AV node faster, if you can get that
signal there faster, than it would fire, then
you've beat it out. So basically, if you can get
that signal from the SA node over to the AV node,
if this happens in less than one
second, then the AV node is not going to get a chance
to fire before you're already firing for it. So this is the race, right? The SA node is basically
trying to get a signal over there quickly. And these distances
that it has to cover, we said about 2 centimeters
and about 1 centimeter. So what is the math? How does that work out? So you can actually
look up these numbers and it turns out that
if you check it out, these conduction velocities
are really, really fast, right? So it's about 0.5 meters
per second up here. And it's gets even faster
as you get along further. So it's about 2 meters
per second here. So these are the
velocities of the signal, how fast the electrical
conduction system is actually sending along that signal. And those are the distances. So if you think about it,
if you just multiply them, you should be able
to get a time. How long it will take a
signal to get from the SA to the AV node. So we know that the SA node
fires every 0.66 seconds, right? That much we have
figured out already. So the question is can it
get a signal to the AV node before the AV node
fires by itself? Can it get a signal down
there in less than 1 second? You're trying to
beat out this time. And can it get a signal
to the bundle of His in less than 2 seconds? You're trying to beat
out that time as well. So let's figure out. So this math works out to,
let's see, you've got 0.5 meters and you're going to
want to end with a time. So I'm going to put
1 second up here. And you have, let's say, 1
meter is 100 centimeters. So the meters cancel. And you've got 2
centimeters to cover. So the centimeters cancel. So you've got 2 divided by 50. And that's seconds. So it's 0.66 seconds,
plus 1/25 That's 1/25. Let me make a little
bit more space on here. Just so it doesn't
feel as crowded. There we go. So 1/25 of a second is
the same as 0.04 seconds. And that is 0.7 seconds. So, so far the signal has
gotten here in 0.7 seconds. I'm just going to
write that in yellow because this is
the SA node signal. 0.7 seconds. Wow, that's really fast, right? Really fast. Let's see how long it takes
to get to the bundle of His. To get to the bundle of His,
I'll do that math over here. You have to now
add up 0.7 seconds because that's how long it
took to get to that AV node. And then you have to add 0.1
seconds and what is that for? This, my friends, is the delay. This is the delay
of the AV node. Remember, the AV node
creates this delay so that the ventricles
contract just a little bit after the atriums do. So this delay is actually
built into the system. The delay is about
1/10 of a second. And then you have to figure out
how long it takes to travel. So how long does it take to
travel that last little bit. Well, it's going to be going
1 second, it covers 2 meters, and we know that 1 meter
has 100 centimeters. And we know that we're
trying to cover 1 centimeter. So centimeters
cancel, meters cancel. And you're left
with 1/200 seconds. So that's how long it
actually takes to travel. Travel time, you
can think of it as. So that would be 0.005 seconds. So in total, it's now
taking us 0.805 seconds. So this is how long it takes
to get to the bundle of His. So now, let me write that up
here, 0.805 seconds, now we're really happy because we were
able to beat out both the AV node-- and I guess from the
perspective of the SA node, if SA node cells got
happy, that's what they would look like. So it basically gets there
really, really quickly is the point. So 0.7 seconds
and 0.805 seconds. So that explains at least
why you never really see the AV node or the bundle
of His cells firing, right? Now going back up here,
imagine for a second that you actually had a problem. Let's say you actually
had some sort of disease or some sort of
issue with your cells and let's say these
SA node cells gave up. Well, if they gave
up, then no signal would be coming into the
AV node and the AV node becomes your plan B. This
is your plan B. The SA node, of course, that's
your plan A. That's what you're usually doing. But it's nice because
you have this plan B and if one second goes
by without a signal, then the AV node kicks in
and that'll start firing. And you'll have a new heart rate
something closer to 40 to 60, but at least your
heart is beating. Now let's say catastrophe
strikes and your AV node is down too. Well, your bundle of
His is your plan C. And so now if 2 seconds go
by and your bundle of His have not gotten a signal,
then they start firing and your heart rate will be
somewhere between 20 and 30. So these are the
backup mechanisms your heart has to make
sure it always beats. And this is one
of the neat things that your heart has figured
out, to create not just a plan A, but also a
plan B, and a plan C.